To date, modern power semiconductor devices including devices such as Si Power MOSFETs and Si Insulated Gate Bipolar Transistors (IGBT) have been typically fabricated with silicon (Si) semiconductor materials. More recently, silicon carbide (SiC) power devices have been researched due to their superior properties. Gallium Nitride (GaN) semiconductor devices are now emerging as an attractive candidate to carry large currents and support high voltages providing very low on resistance and fast switching times. Standard GaN high electron mobility transistor (HEMTs) and related devices are typically normally on, which means that they conduct current at 0 gate voltage.
FIG. 1 shows a standard Ga-face GaN HEMT structure. Substrate 10 may be GaN, SiC, sapphire, Si, or any other suitable substrate for GaN device technology. GaN layer 14 and AlxGa1-xN layer 18 are oriented in the [0 0 0 1] (C-plane) direction. The conducting channel consists of a two-dimensional electron gas (2DEG) formed in the GaN layer 14 near the AlxGa1-xN/GaN interface. The region between the source and gate is referred to as the source access region, and the region between the drain and gate is referred to as the drain access region. This device is normally on or is a depletion mode device. At 0 gate voltage, the 2DEG channel extends from the source to the drain contact, and the device is in the ON state. A negative gate voltage must be applied to deplete the 2DEG under the gate and thus turn the device OFF.
AlxGa1-xN layer 18 is formed with at least a minimum thickness in order to induce the 2DEG channel. This minimum thickness depends on the Al composition in the AlGaN; lower Al composition increases the minimum thickness. FIG. 2 shows the 2DEG sheet charge density ns versus AlGaN thickness for a number of different Al compositions in structures with and without an AlN layer. For thicknesses above the minimum thickness, ns at first increases with thickness but eventually levels out. For structures in which the AlGaN thickness is less than the minimum thickness, applying a large enough positive gate voltage will induce a 2DEG underneath the gate, but not in the access regions. The sheet charge density ns in this 2DEG increases as the gate voltage is further increased.
It is desirable in power electronics to have normally off devices that do not conduct at 0 gate voltage to avoid damage to the device or other circuit components by preventing any accidental turn on of the device. A desirable enhancement-mode (E-mode) GaN HEMT has two features. The source and drain access regions contain a 2DEG with conductivity at least as large as the conductivity of the channel region when the device is in the ON state. Preferably, the conductivity of the access regions is as large as possible, since this reduces the access resistance, thus reducing the ON-resistance Ron. Also, the channel region underneath the gate should have no 2DEG at 0 gate voltage. A positive gate voltage is therefore required to induce a 2DEG in this region and thus turn the device ON.
Further methods and devices that improve an e-mode GaN HEMT access region conductivity while maintaining a gate region with no 2DEG at 0 gate voltage are desirable.